The present invention relates to an image reading apparatus for obtaining image data for an electro-photographic image forming apparatus, for example, and an image forming apparatus having the image reading apparatus.
An image reading apparatus or a scanner converts a reflected light obtained by lighting a reading object such as a sheet-like document, book and three-dimensional object, by an image reading sensor such as a CCD sensor, and outputs the image data.
In many cases, in an image reading apparatus, a reflected light from a reading object is transmitted to a CCD sensor (illuminated by an illumination unit), by moving an optical element set including an illumination unit and mirrors along a glass plate, while remaining a reading object standstill at a predetermined position in a document holding unit (a transparent glass plate).
An illumination unit has a tubular exposing lamp formed to be able to illuminate the reading width that is defined as a total length of an optional one side of a transparent glass plate of a document holding unit.
For example, in a scanner using a halogen lamp as an exposing lamp, a fan is required for cooling a glass plate or a document holding unit, a marker plate which provides a white reference for shading correction, and the inside of a scanner. In a scanner using a xenon lamp or a cold cathode-ray tube with small amount of heat, a cooling fan is not necessarily required.
Although a lamp with small amount of heat such as a xenon lamp is used, a higher illuminance is required to meet an apparatus with a high reading speed (and image forming speed) or a color copying apparatus, and the lamp is heated to a high temperature. In a case where a specific position in a document holding unit is illuminated like in a sheet-through system, the temperature of the document holding unit rises high, and cooling is necessary.
Cooling or ventilation with a fan is necessary also to prevent adhesion of dust to mirrors or the glass surface.
It is known that a lamp represented by a xenon lamp with a phosphor applied to the inside of a tube changes in the light-emitting efficiency of the phosphor, when the temperature in the tube changes. Particularly, when the temperature in the tube is lowered by cooling, the light-emitting efficiency increases and the output light quantity increases. Thus, when a fan is mounted simply for cooling, the luminance at an optional position in the reading width defined in a document holding unit changes in the length direction of the lamp depending on the direction and amount of the wind from the fan.
When illuminance varies in the reading width of a document holding unit, the image quality (level) of an image to be read lowers. The S/N ratio of an image signal outputted from a CCD sensor is deteriorated, and/or variations occur in the shading correction.